The present invention relates to an optical information recording medium on and from which information is written and read by laser beam irradiation and, more particularly, to a phase change optical disk.
Because of the advancement in research and development on the semiconductor laser (to be referred to as LD hereinafter), a blue LD with a wavelength of about 380 nm to 430 nm will soon be put into practical use. The recording density of an optical disk is determined mainly by the focused beam spot size of an optical beam used for writing/reading information. The focused beam spot size is proportional to the wavelength of the LD. Hence, it is expected that the recording capacity of an optical disk will increase greatly by using a blue LD with a wavelength shorter than that of a red LD which has currently been put into practical use.
When a focused beam spot size decreases, the time required for the spot to pass through a certain point on the disk decreases. In a phase change optical disk, this means that the time (holding time) during which the recording layer is held at a crystallization temperature or more decreases.
FIG. 2 shows a result obtained by studying the relationship between the holding time and the spot size by thermal simulation. The holding time is the time during which the recording layer is heated to a range of a crystallization temperature (e.g., 180xc2x0 C.) or more to a melting point (e.g., 600xc2x0 C.) or less. From FIG. 2, it is apparent that the spot size and holding time are almost proportional. Hence, if a blue LD is employed, a recording layer with a higher crystallization rate is required.
In a recording layer with a higher crystallization rate, the stability of the amorphous state is usually low. When the disk is stored at room temperature or the like, data recorded by setting the recording layer in the amorphous state may be undesirably erased (crystallized) over time or upon a change in storing condition.
In order to improve the erasure rate when the blue LD is employed, Japanese Patent Laid-Open No. 11-167747 (reference 1) proposes a technique for adding a light absorption adjusting layer. With the technique described in reference 1, a new layer must be added to the conventional phase change optical disk, leading to a decrease in productivity. With this method, when the linear velocity (rotational speed of the disk) is increased in order to improve the information transfer rate, crystallization does not progress sufficiently and the erasure rate decreases.
It is an object of the present invention to provide a phase change optical disk with a high signal quality, which does not require a particularly new recording layer but has a high erasure rate even when a blue LD is used even at a high linear velocity.
In order to achieve the above object, according to the present invention, there is provided a phase change optical disk with a multilayered structure obtained by sequentially forming at least a lower dielectric layer, recording layer, upper dielectric layer, and reflective layer on a substrate, wherein a reflectance Rc of the disk with respect to light with a wavelength of 380 nm to 320 nm, when the recording layer is in a crystalline state, is lower than a reflectance Ra of the disk when the recording layer is in an amorphous state.